The present invention relates to an automatic focusing device that is employed in, for example, a photographic camera.
In recent years, a camera equipped with an AF (automatic-focus) function is on the increase, and, on single lens reflex cameras with interchangeable lens as well, the AF function has become indispensable. In general, in single lens reflex cameras, a so-called phase difference detecting method is adopted for automatic focusing. The AF with the phase difference detecting method is executed with steps such as the following:
Firstly, a pair of object images with a spatial parallax are projected, respectively, on a pair of photosensitive units, such as a CCD (Charge coupled device), etc., and the light amount received by the respective photosensitive unit is integrated in terms of time. Then, according to the phase differential of two object images on respective photosensitive units, the distance differential between the sensing element (film equivalent plane) and the imaging plane (focus position) of the photographing lens with respect to a photographing object, and the direction thereof (defocus amount/defocus direction) are calculated.
From the calculated defocus amount and direction, drive amount of a motor necessary to drive the photographing lens to make the imaging plane coincident with the film equivalent plane is obtained, based upon which the focus lens is driven along the optical axis thereof. The number of pulses applied to the motor in the above operation is obtained according to the following formula: EQU P=Kv.times.D
Where, P is the number of pulses applied to the motor,
D is the defocus amount, and PA1 Kv is a so-called lens-movement-amount conversion coefficient (K value) which is a coefficient representing the relation between the defocus amount and the number of pulses to drive the motor as necessary to make the defocus amount zero, and is the value inherent to the respective lens.
FIGS. 30 through 32 explain a conventional AF system, as above described. In each drawing, "object image position" indicates the imaging plane of the photographing lens with respect to the photographing object with the position of the focus lens taken as a reference, and "focusing position" is the film equivalent plane, also with the position of the focus lens taken as a reference.
In FIG. 30, as a result of a distance measurement executed at time 0, assume that the distance differential between the focusing position and the object image position, i.e., the defocus amount is detected as DO. Then, to make the defocus amount D0 zero (0), the lens is driven. When the photographing object is stationary or standing-still, the focusing position becomes consistent with the object image position by the results of the driving of the lens. Under this state, interrupting processing of release ON is executed, and an exposure starts after elapse of a release-time-lag t2 which is the time required for mechanical operations for mirror ascent and stopping down of aperture. During exposure, as illustrated in FIG. 30, the focusing position and the object image position remain consistent with each other.
However, when the object is moving (more particularly, moving in the lens drive direction), even if integration and computation are once carried out during its movement, as the object keeps moving while the lens is being driven according to the results of such integration and computation, further integrations, computations and resulting lens drives must be repeatedly executed to keep the focusing position and the object image position consistent.
FIG. 31 shows the case wherein a photographing object is moving from a remote field to a near field at constant speed. The amount of movement of the object image position becomes larger as the object is closer to the photographing lens.
Assuming that distance differential between the object image position and the focusing position, i.e., the defocus amount, at point 1 is D1. When the lens is driven by an amount corresponding to D1 and after elapse of time t1, defocus amount D2 is obtained at point 2. In the same manner, the lens is driven for the amount corresponding to D2, and after the elapse of time T2, defocus amount D3 is obtained at point 3. Here, the focusing position at point 2 corresponds to the object image position at point 1, and since the object keeps moving while time T1 elapses, the defocus amounts would be, EQU D1&lt;D2&lt;D3.
Thus, the defocus amount gradually increases each time when the distance measurement is executed, while the object is moving towards the photographing lens at a constant speed. Therefore, the lens drive can not sufficiently follow the movement of the object image position.
In order to overcome the above problem, the above delay should be prevented by predicting the amount of movement of the object image position from the start of integration to the completion of the computed lens drive, with which the lens is additionally driven. In this case, it is idealistic that the focusing position could be coincident with the object image position when the lens drive by the total amount is completed.
However, even though the idealistic prediction is done, and thereby, the focusing position becomes coincident with the object image position at the time of completion of the lens drive, the object image position may be out of the focusing position at the time when the exposure starts. That is, even if the release ON interruption processing is executed at the point where the focusing position coincides with the object image position, as illustrated in FIG. 32, a defocusing still occurs due to the release-time-lag previously mentioned.
Further, as in the case illustrated in FIG. 32, if the speed of movement of the object image position in the axial direction of the photographing lens (hereinafter called "object image speed") is accelerated, considerable a rear-focusing occurs. In general, comparing front-focusing and rear-focusing defocus amounts with respect to a film equivalent plane, defocusing of the rear-focusing with respect to the actual photographing object is larger than that of the front-focusing. Accordingly, rear-focusing should be avoided as much as possible. For this purpose, it is conceivable that the lens is driven in preemption of equivalence of the release-time-lag, as described above. However, if this preemptive drive is carried out indiscriminately, there would be the case, as the photographing object moves away from the camera, etc., that the object image position might not reach the predicted position after an elapse of the release-time-lag, resulting in a considerable rear-focusing.